Effects Of Drought On The Radial Growth Of Pinus Yunnanensis In Southwest China

Under the background of global warming,the climate presents obvious characteristics of warming and drying and the extreme drought events occur frequently in southwestern China,which seriously affects the growth of trees and forest in the region.However,the effects of warming and drying and frequent extreme drought events on the radial growth of Pinus yunnanensis,the dominant tree species in the region,have not been accurately assessed.Climate warming and drying in southwest China in recent years provided a good opportunity and case for revealing the interaction between extreme drought and forest growth in the context of climate change.We studied the response characteristics of P.yunnanensis to climate factors at different climate zones,altitudes,and age classes on the Yunnan Plateau,and the dynamic transition process of the growth-climate response with warming and drying based on the basic principles and methods of climatology and dendrochronology.The ecological resilience index was used to evaluate the spatial and temporal characteristics of the response of P.yunnanensis to extreme drought events,to explore the internal and external environmental factors that affect the ecological resilience of P.yunnanensis and further reveal the response mechanism of P.yunnanensis growth to extreme drought.We further evaluated the extent of growth inhibition and the resulting growth decline process under the regional aridification.The research on the above issues can predict the health threshold of the regional forest,provide basic data for forest growth dynamics and health assessment under the background of aridification,and provide a reasonable theoretical basis and reference for the rational selection and formulation of regional forest management measures.The main results of this paper are as follows:1.Affected by global climate change,the climate of the distribution area of P.yunnanensis showed obvious characteristics of warming and drying.The increase in frequency and duration of extreme drought events are the main characteristics of aridification of the Yunnan Plateau.The increase in temperature,decrease in precipitation,and spatiotemporal heterogeneity of precipitation are the primary cause for the enhance of aridification of the study area.2.On the regional scale,radial growth of P.yunnanensis is restricted by moisture and drought stress,the interpretation rates of temperature and moisture factors on growth are 52.7%and 47.3% respectively.Radial growth of P.yunnanensis in cold-dry zones were restricted by both energy and moisture conditions;the explanation rate of moisture on the radial growth of trees in hot-dry zones has been up to 90%,moisture become the main factor that restricted the growth;P.yunnanensis in warm-humid zones were restricted by excessive water,the explanation rate of moisture on the radial growth has up to 81.1%;effects of moisture and temperature on the growth of P.yunnanensis in warm zones were relatively balanced.The growth of P.yunnanensis at low altitude were restricted by drought stress induced by high temperature.As altitude increases,the restriction of high temperature on growth weakened,and on the contrary of low temperature effects.The young and over-mature P.yunnanensis trees are more sensitive to moisture,while the near-mature and mature trees are more sensitive to temperature.3.The response model of P.yunnanensis to climate warming and drying is highly dependent on the climate characteristics of the local growing environment.The response characteristic of P.yunnanensis to climate have changed due to warming and drying.P.yunnanensis trees living in cold environment become increasingly sensitive to temperature,while trees become increasingly sensitive to moisture and gradually restricted by high temperature in warm and hot-dry regions with warming.On regional and altitudinal scales,the promoting effects of warming on the growth has become weaker and weaker as the response sensitivity of P.yunnanensis to temperature weakened.Radial growth of P.yunnanensis in low-temperature environment would benefit from warming,while those in high-temperature and water-stressed environment would be constrained by warming.Climate change is more likely to change the sensitivity of P.yunnanensis growth to climate factors under drought stress conditions.4.The resilience of P.yunnanensis highly depends on the climate characteristics of the local environment.P.yunnanensis trees living at the suitable hydrothermal conditions showed stronger resistance to extreme drought,which living in drought-prone areas and close to the growth limit environment showed stronger recovery capability.At the same extreme drought events,the resistance of P.yunnanensis enhanced and the recovery weakened with the increase in altitude gradient,the young and middle-age trees showed the strongest resistance,the near-mature trees showed the strongest recovery.The proportion of P.yunnanensis with high resistance increased significantly but the proportion of P.yunnanensis with high recovery did not fluctuate significantly at the region.Growth amount before drought event,growth similarity among trees and drought intensity were the main factors affecting the resilience of P.yunnanensis in the study area.5.The degree of growth inhibition and decline of P.yunnanensis varied in different climate zones.P.yunnanensis trees living at the cold-dry and hot-dry zones are more prone to growth inhibition and decline than those living at the warm-humid and warm zones.Drought was the main reason for the inhibition and decline of the radial growth of P.yunnanensis in the study area.